Exercise normally causes activation of a portion of the body's reflex nervous system called the sympathetic nervous system, which results in increased heart rate and blood pressure. This study will examine whether leg exercise training will change the reflex responses to leg exercise between three groups of volunteers: young healthy subjects, older healthy subjects and heart failure patients. This will enable the investigators to examine the effects of age on healthy people, and the effects of heart failure in older people. To assess changes in the reflex response, the investigators will measure blood flow, substances produced by exercising muscle (metabolites) and sympathetic nerve activity.